After 3 hours, Plane A has flown (425 mi/h)*(3 h) = 1275 mi.
After 3 hours, Plane B has flown (375 mi/h)*(3 h) = 1125 mi.
Since the distances flown are in directions that are at a right angle to each other, the Pythagorean theorem can be used to figure the separation. The distance between planes after 3 hours is
√(1275² +1125²) = √2,891,250 ≈ 1700.4
After 3 hours, Plane A is about 1700 miles from Plane B.
